A number of different wireless communication techniques have been developed, including frequency division multiple access (FDMA), time division multiple access (TDMA) and various spread spectrum techniques. One common spread spectrum technique used in wireless communication is code division multiple access (CDMA) signal modulation in which multiple communications are simultaneously transmitted over a spread spectrum radio-frequency (RF) signal. Some example wireless communication devices that have incorporated CDMA technology include cellular radiotelephones, PCMCIA cards incorporated within portable computers, personal digital assistants (PDAs) equipped with wireless communication capabilities, and the like.
In CDMA technology and other wireless communication technologies, frequency tracking loops are often implemented to monitor the frequency of received signals and adjust the signals accordingly. In particular, frequency errors (or frequency variations) often exist in the carrier signals received over forward or reverse links of the system. A forward link (sometimes referred to as a “downlink) refers to a signal sent from the base station to a wireless communication device (WCD). A reverse link (sometimes referred to as an “uplink”) refers to a signal sent from the WCD to the base station.
There are generally two main causes of the errors that can contribute to unwanted frequency variation of a carrier signal. The first relates to what is commonly known as the “Doppler effect” or “Doppler shift.” The Doppler effect manifests as a change in the frequency of a received signal due to a relative velocity between the transmitter and receiver. Thus, if a WCD is moving away from the base station as it transmits a signal over the reverse link, the base station receiver will receive a signal that has a lower frequency, i.e., longer wavelength, than the originally sent signal. Because WCDs are often used within vehicles or high speed transit systems, correcting for Doppler shifts can be an extremely important factor in maintaining a robust and effective wireless communication system.
The second cause of error that can contribute to unwanted frequency variation relates to variations between local clocks of the various devices in the wireless communication system. Each device in the system typically produces carrier signals using a frequency synthesizer that utilizes the local clock of the device as its timing reference. Each local clock, however, typically has an unknown timing error. WCDs, in particular, often implement relatively low-cost local clocks, such as voltage-controlled, temperature-compensated crystal oscillators (VCTCXO). These local clocks can introduce significant frequency errors in the carrier signal.
To account for frequency errors and adjust the signals accordingly, frequency tracking loops are often implemented within WCDs and base stations in wireless communication systems. For example, a frequency discriminator can be used to compute an estimate of the frequency error. In particular, the frequency discriminator calculates residual frequency error estimates and sends them to an accumulator that continuously accumulates the residual frequency error estimates to estimate the actual frequency error. The accumulated estimate is used by a rotator to adjust the frequency of the received signal accordingly, thus reducing the residual frequency error. The residual frequency error eventually converges to approximately zero, such that the accumulated estimate is approximately equal to the actual frequency error. In this manner, a feedback loop can correct for frequency errors in a received carrier signal.
There are generally three frequency tracking loop variables that characterize the performance of the frequency tracking loop: the time constant of the loop, the standard deviation of the residual frequency error, and the pull-in range. The time constant is an estimate or measure of the time it takes for the frequency tracking loop to reduce the residual frequency error to approximately zero after the loop has been initialized. Initialization of the loop can occur, for example, when a finger of a RAKE receiver is initially assigned to a path. The pull-in range is a strict upper bound defining the maximum residual frequency error that can be handled by the frequency tracking loop. For example, upon initialization, if the residual frequency error is greater than the pull-in range, the frequency tracking loop will not converge.
In general, it is advantageous to minimize the time constant, minimize the standard deviation of the residual frequency error, and maximize the pull-in range. In particular, the time constant and the standard deviation directly affect system performance in the sense that they affect the error rate of the system. The pull-in range typically limits the relative velocity at which a wireless communication device will still be able to effectively communicate in the system. Often, these three variables represent trade-offs to designers of wireless communication systems.
Pilot symbols are often used to estimate the residual frequency errors. Pilot symbols can be extracted from a received signal by demodulation, e.g., despreading and Walsh/OVSF decovering. Pilot symbols generally refer to the control symbols that have been conventionally used to facilitate system synchronization. Pilot symbols are typically transmitted over a control channel. To compute an estimate of the residual frequency error, a first pilot symbol can be cross-multiplied with the complex conjugate of a second pilot symbol to calculate phase rotation between the successive symbols. In this manner, an estimate of the residual frequency error can be obtained and then accumulated.
For some wireless communication systems, other control symbols, i.e., non-pilot symbols, have been implemented within the control channel to improve system performance. For example, in W-CDMA, both pilot symbols and non-pilot symbols are transmitted over the control channel. This presents challenges to the effective implementation of frequency tracking loops. In particular, achieving the desired design goals in W-CDMA for the standard deviation of the residual frequency error and the time constant can result in a small pull-in range. To compensate for this small pull-in range, complex searcher elements are conventionally required to search over multiple frequency offsets to ensure that the residual frequency error falls within the pull-in range. Still, even with these complex searcher elements, the frequency tracking loop may not converge if a wireless communication device is traveling at very high velocities, such as velocities associated with high speed transit systems.